In automatic washing machines, DC brushless motors have conventionally been used as motors for rotating a mixing blade (pulsator) or a rotating tub when washing, rinsing and dehydrating operations are carried out. A system for driving the DC brushless motors by an inverter circuit has been employed widely. Voltage applied to the motor is increased or decreased when torque is controlled according to driving conditions of the motor. However, output torque controlled by the applied voltage is not proportional to voltage although a rotational speed of the motor is proportional to the output torque. As a result, a difference tends to be caused between a target speed command and a detected motor speed, whereupon the control tends to become unstable.
In view of the foregoing problem, the inventors proposed in Japanese Patent Application No. 2001-171185 the technique of controlling the output torque by a q-axis current by applying a vector control in order that the motor used in the washer or the like may be controlled with higher accuracy. More specifically, motor torque control and accordingly rotational speed control can be carried out with high accuracy since the output torque of the DC brushless motor is proportional to the q-axis current obtained by the vector control.
Motors used in the washers or washer-driers are controlled so as to be rotated at low speeds in the washing operation and so as to be rotated at high speeds in the dehydrating operation. Since induced voltage caused in a motor winding is raised during high speed rotation, drive voltage to be supplied also needs to be raised with rise of the induced voltage. Generally, however, the drive voltage which can be supplied has a definite limit when an electric motor is controlled by an inverter. Accordingly, a field weakening operation is carried out in the high speed rotation so that the number of revolution of the motor is increased while the induced voltage of the motor is restrained.
In the vector control technique proposed by Japanese Patent Application No. 2001-171185, an operation is performed according to a given speed command value and detected motor current value, whereupon drive voltage applied to the inverter circuit or the duty of PWM signal is determined. Thus, a level of actual drive voltage is unknown until the results of operation in the vector control are obtained. Accordingly, in the case where the field weakening operation is performed while the vector control is carried out, the d-axis current is changed from a full field (=0) state thereby to be set at a smaller value when it is determined that the PWM duty exceeds 90% (namely, 90% of the drive voltage).
More specifically, output voltage of an inverter circuit cannot be set to be approximately 100% when the field weakening operation is carried out. Accordingly, since the magnetic field needs to be weakened more as the voltage drops, the motor efficiency is reduced and the power supply current is inevitably increased. As a result, the inverter and motor become large-scale.
The present invention was made in view of the foregoing and an object thereof is to provide an inverter which can raise the output voltage in the case of the field weakening operation even when the vector control is introduced to the control of the motor used in the washer or washer-drier.